Many Collins Radio enthusiasts believe the 51J Series comprises the
best looking receivers that the company produced - but, what about
51J performance? Like many pieces of collectible vintage electronic gear,
the 51J receivers are now over half-a-century old. Many users are
operating their 51J receivers in "as-found" condition. Are they
experiencing all that the 51J is capable of? The following article
details how to get the most out of your 51J receiver. Also, a look at
some of the common problems encountered when delving into any of the 51J
receivers. I've included lots of photos and detailed descriptions for
rebuilding or repairing. As some of the best-looking receivers that
Collins produced you definitely want your 51J to perform like that
company's best equipment should. H. Rogers, Dec. 2013

photo above: 1950 advertisement for the
Collins 51J-1 Receiver

Brief History of the 51J Receivers

Introduced in 1949, the Collins 51J receiver was
intended for the military or commercial users that required an extremely
accurate frequency readout and a very stable, drift-free receiver that
was especially suited for use in data reception, such as RTTY, but could
also provide excellent reproduction of AM or CW signals. The 51J was
also the perfect receiver for the civilian enthusiast - if he could afford its
nearly $900 price tag. Collins Radio Company had entered into the ham
radio receiver market
with their ham band only 75A-1 receiver two years earlier, but their 51J-1 was
for commercial/military users and was Collins' first receiver to provide
general coverage reception from .5mc up to 30.5mc with better than 1kc
dial accuracy anywhere within that tuning range. The tuning system used
thirty, 1.0mc wide tuning ranges and the receiver itself featured
performance that was literally years ahead of the competition.

During the post-WWII
era, the U.S. Army Signal
Corps was supposedly trying to use some of the WWII
vintage receivers for RTTY and were finding the drift to be excessive
for RTTY and the dial accuracy to be inadequate. These receivers have
been reported to be the BC-348 but the BC-342 seems a more likely choice. The story goes that Collins sent
an early 51J receiver to the Signal Corps to try out and the Signal
Corps' response
was certainly positive since the receiver was extremely accurate in its
frequency readout and had essentially "no drift." The SC "had
to have 'em" and ultimately the R-388/URR was the result. It seems far
more likely that Collins was working with the Signal Corps from the
beginning to provide them with a thoroughly modern receiver that had the
specifications the military sought. From the start, the military bought
51J-1 and 51J-2 receivers designated as R-381/URR receivers (non-A and A
versions respectively) but the ultimate design came in late-1950 with the R-388/URR receiver. Many thousands of R-388
(also designated as 51J-3) receivers were
built by Collins from 1950 up to about 1962. The R-388 was found in many frequency
diversity RTTY set-ups used by the military, many times installed in portable
communications huts. The 51J Series wasn't entirely a military receiver
however. Other commercial users found a need for the precision tuning
and frequency stability. The 51J-4 version,
with its stability and dial accuracy along with its mechanical filters, was found in
coastal stations like KPH and KMI, in overseas embassies, in commercial
laboratories like Beckman, at universities like Stanford, along with some military uses
and even some wealthy SWL (supposedly Jackie Gleason owned a 51J-4) and ham enthusiasts bought 51J-4 receivers.
The 51J-4 was in
production until 1964 with a production of at least 7500 receivers
(that's about how high the 51J-4 serial numbers go.) All total, with the
early 51Js, the R-388s and the 51J-4 included together, the production
probably was over 20,000 receivers making the 51J Series one of Collins'
most produced receiver.

Circuit
and Design Description

51J-1 and 51J-2

The 51J receiver utilized a
permeability tuned, double conversion circuit using the 70E-7 PTO in a
dual, tuned-IF system and a multiple frequency crystal oscillator to
cover .5mc to 30.5mc in thirty (1mc wide) bands. The tunable dual IF
system selects either 1.5 - 2.5mc (even) or 2.5 - 3.5mc (odd) depending
on whether the band number selected is odd or even. The band select
numbers start with .5 to 1.5mc being Band One (odd,) 1.5 to 2.5mc being
Band Two (even,) etc., on up to 29.5 to 30.5mc being Band 30 (even.) Band 1 actually is triple conversion but
only to allow coverage of the AM BC band. Bands 2 and 3 are single
conversion since they are essentially the frequency coverage of the
tunable dual IF system. All of the remaining bands are double conversion and
select either the even or odd tunable IFs as required in combination
with the Crystal Oscillator and PTO to provide the correct tuning range. The fixed IF is
tuned to 500kc and has three amplifiers. A Crystal Filter is provided
for adjustable selectivity and heterodyne interference reduction. A standard envelope detector, AVC and a Noise Limiter are
also included. 16
tubes are used in the 51J-1 and J-2. The ham bands are high-lighted in
green on the megacycle drum dial but, at $875 list price, not many hams
could afford a 51J as their station receiver.
Incidentally, the $875 price did include a table top cabinet for the
receiver, if so ordered. Otherwise, a rack mount configuration was
supplied.

Early versions of the 51J receiver have a metal dial bezel, the 70E-7
PTO (with the 6SJ7 metal octal tube located under a screw-mounted shield
cap,) the Collins' "winged emblem," no grab handles and an illuminated
S-meter. Audio response is restricted at 200 to 2500Hz and is definitely
not high fidelity, usually sounding somewhat "muffled" when receiving AM
voice signals. The most apparent difference between the 51J-1 and 51J-2
is that the latter added an Audio Output function to the new dual scale Carrier Level
meter that was actuated by a toggle switch next to the meter. Another
difference is the nomenclature for the 100KC Crystal Calibrator, was
designated as "100KC CRYSTAL" on the early 51J-1 and merely as
"CALIBRATE" on most 51J-1s and all 51J-2s. Some later production 51J-2 receivers may be found with the
70E-15 PTO (round can and two 6BA6 tubes) installed but whether this was
a post-sale retrofit or a Collins-upgrade is
unknown at this time. >>>

photo above: The Collins 51J-2 from 1950 in the standard Collins
cabinet

>>> It's
also possible that the green high-lighting of the ham bands on the
megacycle drum dial was eliminated in the later 51J-2 receivers since it
was unlikely that sales to hams accounted for many purchases. Both the
70E-15 PTO and the standard megacycle scale drum are on the standard
production of the 51J-2's successors, the 51J-3 and R-388/URR, along
with several other changes in the receiver circuit and in its
appearance. The 51J-1 was built in
a very small quantity in 1949 and by the end of that year it had been
replaced with the 51J-2. The J-2 was built through most of 1950. The
military ordered both 51J-1 and 51J-2 receivers and they were designated
as the R-381/URR and R-381A/URR. Only a relatively small quantity of
51J-2 receivers were produced before the upgraded version 51J-3 replaced
it.

R-388/URR

In late 1950, the 51J-3/R-388/URR was introduced, featuring an 18 tube circuit (adding a voltage
regulator 0A2 and VFO buffer 6BA6,) a new version of the PTO (70E-15)
and eliminating the fixed 300 ohm Z antenna input (by removing the
primary winding on the antenna coils) and redesigning the antenna input
to a more flexible design utilizing an Antenna Trim control. This revision was
probably at the request of the Signal Corps, who wanted to use the new
version of the receiver for
their RTTY installations but found old 51J antenna requirements of a
fairly Hi-Z, "fixed" (non-adjustable) 300 ohm antenna input impedance
difficult to work with since most of the Army installations used
either 75 ohm Z dipoles or 50 Z ohm (or less) vertical whip antennas.
The new upgraded receiver was designated as the R-388/URR (when going to
the Signal Corps) and it was built from 1950
through about 1954 in relatively large quantities. There were also later
contracts for 1955, 1956, 1957 and 1962 but these later contract quantities
only total about 1000 receivers. From 1950 through 1962 at least 12,000
R-388/URR receivers were produced (the actual total may be somewhat
higher.) All contracts were built by Collins Radio Company although
in the early 1950s Hallicrafters supplied a special cabinet (along with
a manual) that was utilized by the Navy for some of it's R-388
installations. Some of the U.S. Navy R-388/URR receivers were designated as AN/URR-23-A (but
possibly tagged as 51J-3) and this set-up included the receiver installed in a Collins table cabinet
(designated as CY-1235/U) and
a Collins 270G-3 speaker (designated as LS-199/U.) Additionally, the USAF had Raytheon supply them with a few receivers that were designated
AF-30 and were used for backscatter ionospheric purposes. These USAF
receivers
had blue panel and lacked grab handles. The largest contract quantities
for R-388/URR receivers are from 1951, 1952 and 1953 with these receivers
being the most commonly encountered today.

With the R-388 and 51J-3, grab handles were added to the front panel
along with a high quality Burlington Co. sealed meter (some receivers
will be found with different makes of meters but they all have similar
scaling.) The conventional "Remote Relay" function that paralleled the
front panel "STAND BY" and "ON" functions found on the 51J-1 and 2 was
eliminated in the R-388 receiver. Instead a "Break In" function was
added that utilized +12vdc
to actuate an internal relay that disconnected the antenna and removed the IF plate
voltage upon actuation. Later, there was also a military supplied modification kit for the
end-user addition of a solid-state RF-driven relay circuit to protect
the receiver's antenna input circuits from high levels of antenna
current induced from nearby transmitters while the receiver was in
operation (break-in off.) This replaced the neon bulb
antenna static-drain protection device. >>>

>>> All
R-388 or 51J-3 receivers now had
the entire underneath of the chassis protected with a full bottom cover
rather than the 51J-1 and 2's small cover that was just over the
receiver's front end. The metal bezel used on the J-1 and J-2 was
replaced with a black bakelite bezel. There are many other changes
in component types and placement throughout the R-388 receiver
making it quite a different receiver from its predecessors, the
51J-1 and J-2.

Late versions of the R-388, usually the 1953 and later
contracts, have a switch to turn the "Break-in" on or off
(with external +12vdc supplied.) This
switch replaced the SPEAKER jack and mounted in that location of
the panel. Many times a MWO (Military Work Order) will have been
incorporated to add the Break-in Switch or other changes.
Most production quantity totals are listed as "for the R-388 receiver" with 51J-3
production not specified. The totals actually may be for both receivers produced
and stocked by Collins, later to be used to fulfill contracts to
the Signal Corps (as the R-388) or the USN (as the R-388 or 51J-3.)

Though the standard carrier level meter was supplied by
Burlington, it isn't unusual to find that the original meter
will have been replaced during a field repair. The Marion
Electric type of carrier level meter is shown in the 1951
contract R-388 in the photo to the left. The 1953 R-388 shown below has
the MWOs listed on the front panel. Also, note the Signal Corps
acceptance stamp near the meter.

The 51J-3 is virtually identical to the R-388/URR
receiver. Some "R-388" receivers supplied to the USN were actually
identified as "51J-3" on their data plates (which were Collins serial
number plates.) Civilian 51J-3 receivers are seldom encountered.

Shown
to the right is a U.S. Navy
receiver that is identified as a "51J-3" on the data plate. On the rear of
the chassis is "vibra-etched" the following, "ERL-C-283" followed by
"USN-091734-049424." Also, ink-stamped on the rear chassis is "MFP
JAN-1953." Additionally, the Collins' Production Serial Number "2106" is
stamped into the metal on the rear chassis and the same number is white ink-stamped on the
front panel under the data plate. There is an "anchor USN" acceptance
stamp located on the rear of the meter. The serial number on the data
plate, "12042," is a very high number and implies that this SN might not
be tied sequentially to this specific contract since the supplied
quantities of R-388 receivers on all contracts is estimated to be
just somewhat over 12,000 receivers (see update below.)

It appears that the MFP was
applied after the Collins' Production SN was stamped which would date
receiver build-date before January 1953. The filter capacitor is dated-coded
1952 but this isn't a very reliable manufacturing dating source. When the "built" receiver was pulled from
Collins stock to be sent to the Navy the data plate was stamped,
serialized and mounted.

This 51J-3 receiver is in very good condition both cosmetically and
operationally. It's installed in a typical Collins table cabinet. The photos below show the various identifications found on
this USN 51J-3. The data plate is a reproduction - see update below.

photo above: Collins 51J-3 repro data plate

photo above: MFP date, USN
identification and Collins' stamped SN

photo right: USN "Anchor" acceptance stamp on Carrier
Level meter

UPDATE: August 23, 2018 -
12,000+ Serial Numbers on 51J-3 tags - I received an e-mail from
Tom N5OFF regarding these high serial numbers on 51J-3 receivers. About
20 years ago, Tom obtained permission from Collins to reproduce the
51J3/4 serial number data plate. To assure Collins collectors would be
able to easily identify these high quality, authentic but new data
plates as "reproductions," Tom stamped the series of 200 tags with
serial numbers beginning with 12000.

Of course, this information puts
into question the origin of the receiver profiled above. In all
probability, this receiver is a standard pre-1953 production R-388. It
is the second R-388 that I know of that was sold as a 51J-3 receiver. Be
very careful when inspecting any 51J-3 for intended purchase. Flags such
as military inspection stamps, MFP coatings, etc. should be called into
question.

51J-4

In 1955, the 51J-4, with 19 tubes and three mechanical filters (1.4kc,
3.1kc and 6.0kc actual bandwidth,) became available and was offered up to
about 1964. The 51J-4 was to be the ultimate evolution of the 51J
Series. Collins had recently developed the mechanical filter that
provided an IF passband that was defined with steep skirts and a flat
top that resulted in superior selectivity. Interference from adjacent
frequency signals would just disappear as the IF passband was narrowed
as more selective mechanical filters were switched in. As HF band
congestion increased, so did the need for a more and more selective
receiver. The 51J-4 added a fourth stage of IF amplification to
compensate for the insertion loss of the mechanical filters. It will be
noted when inspecting a 51J-4 chassis that there is a Mechanical Filter
Assembly that is mounted to the chassis. Under the MF Assembly, the
chassis is punched for the R-388 type of IF construction and has the
silk-screened nomenclature for the R-388 also. The addition of the
MF Assembly Unit includes two 6BA6 tubes mounted on top of the unit that
actually amplify the input and output of the selected mechanical filter
thus bringing the total number of IF amplifiers to four although there
are only three "fixed-tuned IF amplifiers." The mechanical filter selector switch shaft
has a chrome level that is placed behind the BFO knob. The factory 51J-4
receivers will have the MF bandwidth silk screened on the front panel as
1KC, 3KC and 6KC.

U.S. Navy catalogs show that the 51J-4
was designated as the R-388A/URR or the R-388B/URR but it is doubtful
that any receivers were actually identified as such. At this time, no
"tagged" R-388A receivers have been found which seems to indicate that
although the designation was used in Navy catalogs, the actual receivers
were "tagged" as "51J-4."

51J Series Summary
- Dial accuracy and stability - both
necessary for RTTY work - were the 51J selling points to the Signal
Corps and still are the 51J's primary attributes today. Although the
stock audio is somewhat restricted, the 51J receivers are great
performers providing they have been thoroughly serviced or rebuilt and
are fully aligned. Although the 51J-4 and R-388/URR are considered the
ultimate design level for the series, the 51J-1 and 51J-2 have their
appeal and can also provide great reception.
The 51J Series, especially the R-388 and the 51J-4, are very popular
receivers for vintage ham stations providing great performance with
fabulous visual appeal. Sensitivity is very competitive and dial
accuracy (for an analog readout) can't be beat. Drift is non-existent.
The fact that the audio reproduction is limited doesn't affect ham
performance very much since most amateur transmitters are already
limited to 300hz to 3000hz audio bandwidth anyway. Even many of the
Shortwave Broadcasters have somewhat limited audio and then, with
variable propagation affecting reception, the 51J's audio limitations
really aren't that noticeable. The 51J audio isn't for
audiophiles,...it's for reliable communications.

General
Information on the Various Types of 51J Receivers

Standard Versions

51J-1 - 1949
- The first 51J model was equipped with an illuminated S-meter, the Collins "winged emblem",
a metal
dial bezel with "51J RECEIVER" silk-screened above the kilocycle dial,
the 70E-7 PTO, the megacycle dial featured green highlighted amateur bands,
"100 KC CRYSTAL"
nomenclature was used for the Calibration Oscillator switch (on the
earliest production only,) the bottom of
chassis had small cover for the RF sections only with the rest of the
chassis bottom uncovered, no grab
handles were used, no skirt was on Megacycle Change knob but it did feature a
retractable crank, a single phone jack on front
panel and the circuit used 16 tubes. The antenna input impedance was fixed at 300 ohms nominal Z
with no Antenna Trim control used. Each Antenna coil had a primary winding on
the J-1 and J-2 receivers.

51J-2 -
1950 - The second 51J models had essentially the same
characteristics as the J-1 with these following exceptions; "CALIBRATE" replaced
"100 KC CRYSTAL" on
panel nomenclature (late 51J-1s had this change,) the meter was changed to a Carrier Level meter with
appropriate scaling but meter was still the square front bakelite
housing model with illumination. A switch was added to the meter circuit
to allow measuring either carrier level or audio output, marked "METER -
INPUT - OUTPUT." Very late versions of the 51J-2 may be found with the
70E-15 PTO. It's also possible that the green highlighted amateur bands
were removed from
the megacycle drum dial sometime during the 51J-2 production.

R-388/URR
- 1950 to 1962 - This is the military version 51J receiver that
featured major changes to the original 51J receiver design. The
antenna input impedance changed from fixed 300 Z ohms to low-Z 50 ohms adjustable by
eliminating the primary winding on the antenna coils and adding an
antenna trimmer capacitor with front panel control. The dial bezel was
changed to a black bakelite piece and "51J RECEIVER" not used anymore on
the receivers, the megacycle dial drum no longer had the green
highlighted amateur bands (this may have been taken off during the 51J-2
production,) the carrier level meter was now a non-illuminated, sealed
unit made by Burlington Company (replacement meters made by Marion
Electric and other companies are sometimes found installed,) audio
outputs on the front panel allowed for phone or speaker connections
(pre-1953,) grab handles were now installed on the front panel, a
skirted-knob used for megacycle change, an 0A2 voltage regulator tube
was installed, a PTO buffer output tube was installed (brought total
tube count to 18,) the entire bottom of receiver chassis was now
protected with a slide-in aluminum bottom cover, many of the aluminum parts now had an irradite finish (gold color,) the
side panels are made of steel and finished with gold color irradite, the schematic
was usually applied to the inside of the top cover, most (all?)
receivers were MFP coated, remote standby now required +12vdc to be
applied to a relay via real terminals for "break in" operation. An
SO-239 connector on rear chassis, "IF OUTPUT" was provided for driving RTTY TUs and other
data devices. 1953 and later receivers have a "Break-in" on-off switch
added to the front panel in the same location as the SPEAKER jack which
was removed. Diode Load and AVC pin jacks are added to the rear chassis
on last of production. The R-388 was supplied to Army Signal Corps at various times
from 1950 up into the early 1960s.

51J-3 - 1951 to 1955
- The 51J-3 is virtually identical to the R-388/URR but the ID tag will
show that the receiver is a 51J-3
rather than R-388/URR. It's possible that when the USN ordered the
AN/URR-23-A the receiver shipped was actually identified on its ID plate as "51J-3." The minor differences between this USN version and the
Signal Corps R-388/URR are that under the top cover there is no schematic of the
receiver (some R-388/URR receiver top-covers also don't have the
schematic, but most do.) There are no Signal Corps acceptance stamps applied to the
chassis or front panel. The Collins' build serial number is ink-stamped
on the front panel under the serial number tag (this same number is
stamped into the rear of the chassis.) Some R-388/URR receivers may have
the build SN under the tag but many have acceptance stamps under the tag
or nothing at all under the tag.
To add to the confusion, sometimes R-388/URR receivers will have Signal
Corps stamps AND Navy "Anchor" stamps on the front panel of the same
receiver. The actual "civilian" 51J-3 is rare.
It's probable that the civilian receivers are exactly the same as the
R-388/URR but should be lacking the MFP coating, shouldn't have any
military inspection stamps and the tag will identify
the receiver as "51J-3." Also, 51J-3 serial numbers in the
12000 range indicate that the data plate is a reproduction and calls
into question the authenticity of the receiver as a true 51J-3.
Sometimes R-388 receivers will have had their military ID plate removed
and then are merely misidentified as the 51J-3. Check for Signal Corps
acceptance stamps or other indicators to verify the correct identification for "tag-less"
receivers. Basically, the civilian 51J-3 and some USN 51J-3 receivers
might have "51J-3" on their ID tag while ALL Signal Corps R-388/URR
receivers will have "R-388/URR" as the receiver identification.

51J-4 - 1955 to 1964
- Civilian, Commercial, Military versions are all the same with the
receiver itself being very similar to the R-388 with the following
exceptions. The 51J-4
added three mechanical filters to the IF by installing a mechanical
filter "kit" into the R-388/51J-3 chassis. Examine the 51J-4 chassis and
it will be seen that the chassis is punched for the standard IF strip of
the R-388 but that a mechanical filter assembly package has been
installed which converts the receiver to a 51J-4. The front panels were
also changed to add 1KC, 3KC and 6KC nomenclature for the mechanical filter selector
switch which is located behind the BFO knob. The MF assembly adds an
additional IF amplifier bringing the tube total to 19. Late version
panels (made in 1964) may be painted light gray with black nomenclature.
Some late versions have been found with "S-line" knobs installed. 51J-4
receivers are usually not MFP coated. AVC and Diode Load pin jacks on
rear chassis. "FCC Part 15" information is silk-screened on the rear of
the chassis on most of production. When supplied to the USN as the
R-388A/URR or R-388B/URR, it's likely that the ID plate has "51J-4"
stamped on it.

Special Versions

R-388A/URR and R-388B/URR -
US Navy version of 51J-4 - it is unknown if this model was actually
produced (or tagged as such) even though it's listed in the Navy books. Same with
the R-388B. These receivers may actually have the 51J-4 designation on
the ID tag.

AN/URR-23 and
AN/URR-23-A - These are Navy designations for the 51J-2
and 51J-3 installed in a Collins cabinet designated as CY-1235/U
with Collins speaker designated as LS-199/U.

R-381/URR and R-381A/URR - These are Signal Corps
designations for the 51J-1 and 51J-2.

AF-60 - Special
USAF version of the R-388/URR with blue panel and no grab handles. Very
small contact of only seven receivers. Used for ionospheric backscatter
research.

Civilian Cabinets
- The cabinet for the 51J receivers were optionally available and are
found with many of the civilian receivers, especially 51J-4 receivers.
The cabinet is similar to any of the late-forties and early-fifties
Collins' cabinets that were used on the 75A-1, 75A-2, 75A-3 receivers
and the 32V-1 and 32V-2 transmitters. A true 51J cabinet will have a
very large rectangular opening at the rear of the cabinet to allow
access to the three wing-nuts that secure the top lid, although, with the standard
75A/32V cabinet the 51J top can't be removed without taking the receiver
out of the cabinet.

Military Cabinets
- Hallicrafters supplied a CY-1260/G cabinet, shown in the photo left, that was used with the
R-388/URR in some applications. Hallicrafters also supplied a manual
when the receiver was installed in this or similar Hallicrafters'
cabinets. Also, there was another Hallicrafters
cabinet made in a similar style to the CY-1260/G that would hold two R-388
receivers. Hallicrafters actually made several CY-version cabinets that
were available for several different types of receivers including the
SP-600 and the R-274D (military SX-73.) These Hallicrafters-built
cabinets have probably led to the myth that Hallicrafters built R-388
receivers which, of course, wasn't the case.

The Hallicrafters-built cabinets are fabricated with heavy steel and
welded seams. Inside an elaborate guide set-up was installed to mate
with metal rolling wheels mounted on the rear sides of the R-388
receiver. This allowed very easy installation and removal of a receiver
from the cabinet since sliding friction was greatly reduced. The rear
cutout allows easy access to the rear terminal strips and SO-239
fittings. Louver vents are on both sides of the cabinet but no grab
handles or slots were provided which makes moving this heavy cabinet
somewhat difficult. Note in the photo to the left that the receiver is
raised off the table by about two inches when using the CY-1260 cabinet
which increases the total height to about 13.5 inches.

photo above: 1952 R-388
receiver installed in the Hallicrafters CY-1260/G military cabinet.
These types of cabinets are built of heavy-gauge steel and their weight
is formidable.

photo right: The data plate on the
Hallicrafters CY-1260/G cabinet

Speakers -
All of the advertising seems to never specify a particular matching
speaker for any of the 51J series of receivers. This seems to imply that
the standard 8" speaker used in the 270G-2 cabinet was used with the
early 51J receivers and the 270G-3 10" speaker was used with the 51J-4.
R-388 receivers were generally used for specific military set-ups such
as RTTY or other data transmissions where a loud speaker would not be
necessary (a headset would be used for monitoring signals.) There was a USN version of the R-388 referred to
as the AN/URR-23-A that came in a standard Collins cabinet, CY-1235/U and included
the 270G-3 speaker (LS-199/U.) The 51J-2 was also supplied in a similar fashion and
designated as the AN/URR-23.

General Information on Reworking 51J
Receivers

Rebuilding Difficulty
- The 51J Series is a fairly difficult receiver to work on because it is
not modular in design and needs to be substantially disassembled to work
on the commonly encountered problems, including those problems involving
the PTO. The front panel always has to be taken off to remove the PTO.
Working on the PTO requires careful attention to the delicate parts
inside. The PTO usually must be calibrated for
minimum end-point error outside of the receiver on the 51J-3, J-4 and R-388. This requires that a jig be made to
accurately measure the turns to better than one degree of rotation. If
work is required in the receiver's front end, it is extremely difficult to access
any of the parts in the crystal oscillator and most of the other front-end stages
aren't much easier to get into. Additionally, the wires from the various coils are very
fine and easy to accidentally break when doing rework in the front-end
of the receiver. All R-388 receivers will be MFP'd which
adds to the difficulty of rework. The 51J gearbox is complex and rework
difficult. To take on a poor condition 51J receiver, you should be experienced in complete disassembly and
reassembly of communications equipment. You should have professional soldering
equipment, possess a good soldering technique and use only real SnPb
solder. Your test equipment should be laboratory-type although your skill at RF/IF alignment will determine the
ultimate quality
of the receiver's performance. The 51J Series is certainly
within the capabilities of most restorers who have some communications
receiver experience. Don't be in a hurry and always be thorough.

Some Thoughts on Rebuilding
- Most of the Collins 51J Series receivers will operate as found.
However, these receivers are probably not operating to their full capabilities
and generally give the new owner a feeling that the 51Js are over-rated. Although
you won't find handfuls of bad capacitors (as in the early Hammarlund SP-600s)
there are several common problems with the 51J receivers now that they
have aged over five decades. Also, since the receivers were used
extensively and then were probably stored poorly, you will usually find some mechanical problems that will
need to be fixed. Once all of the circuits has been rebuilt and the
mechanical issues addressed, a full alignment is necessary. Though not
essential, the original alignment tools will make many of the
adjustments easier. Early 51J
receivers that use the 70E-7 type PTO align easily. Later receivers with
the 70E-15 PTO will almost always require a rebuild of the PTO to function at their
design level of performance. When completed, the 51J Series receivers
are fully competitive with any other mid-fifties communications
receiver. Highly sensitive, very accurate dial resolution and great
selectivity. Note, that high fidelity audio was not included. The 51J
receivers have highly restricted audio since they were primarily
designed for communications, specifically data reception.

Commonly
Found Problems and Component Issues - A
thought for consideration is that most of the earlier 51J receivers, the
51J-1 and J-2, have had thousands of hours put on them by former
commercial/military users. These receivers were sometimes roughly
treated and are usually well-worn, needing much more than the usual
re-cap and alignment to function at their design level of performance.
The early 51J-1 & 2 gear boxes have brass drive gears where the later
R-388/51J-3 and J-4 versions have steel drive gears. Sometimes the wear on
early gearboxes is so severe that binding and jamming are experienced
when changing the Megacycle position.

The early IF transformers often seem to
sustain internal damage with rough handling of the receiver requiring
disassembly and re-gluing of the coils and ferrite shields back into
their proper position for correct operation.

If you are working on a 51J-2 receiver
be aware that the 1950 version of the 51J-2 manual is fraught with
errors in almost every section - almost on every page - but especially
in the receiver alignment section, circuit description section, the
component designations and component identification layouts. How this
manual got through the proof-readers is a mystery. The 51J-1 manual is
probably just as bad. The later R-388 and 51J-4 manuals are much better
with very few errors found. However, the R-388 schematic found in some
manuals has several component identification numbers transposed which
makes correct ID'ing difficult. The schematic that is glued inside the top cover is
correct. >>>

>>> The R-388 and 51J-4 receivers seem to
be in much better overall condition and usually don't require extensive
rebuilding, although there certainly are exceptions. Most component
problems seem to be aggravated by poor storage conditions. However,
there are some component quality issues with some types of capacitors that will affect the AVC
circuit performance. This seems to be a problem mostly encountered with
the earlier receivers and is seldom found in the R-388 or 51J-4.

Most of the 51J problems encountered will
be mechanical in nature and brought on by poor storage, rough handling or
"hamster"
modifications.

Capacitors - Nearly all of the capacitors
used in the 51J front-end and IF sections are tubular ceramic caps which
are extremely reliable and never seem to fail. There are some issues with the small silver mica
caps (the little red ones) in that sometimes they will short but this is rare. Also, the
variable trimmer capacitors are integral to their fiber mounting boards
in the front-end. These can and do "stick." Do not force any stuck
trimmer. First apply some De-Oxit to see if that frees the trimmer. If
De-Oxit doesn't loosen the stuck trimmer than try a bit of heat on the
metal slotted head. Use a small soldering iron to apply the heat. This
will sometimes melt and loosen material that won't dissolve in the De-Oxit.
You don't want to break the trimmer since they can't be replaced without
special
modifications.

The filter capacitor assembly plugs
into an octal socket. These are dual electrolytic capacitors with 25uf
per section used in the 51J-1 and 51J-2 receivers. The R-388 and 51J-4
use either 35uf per section or 40uf per section depending on if the
capacitor
was ever replaced. Working voltage is usually 450vdc on all types.
These are very reliable, well-sealed capacitors that seldom fail. Check
for leakage current before powering up the receiver. Capacitors that
haven't had power applied for decades should be reformed. Leakage current on a reformed
electrolytic should be less than 100 uA at full working voltage.

Unnecessary Modifications
- There are plenty of modifications that have been published for the 51J
Series. A search on the web will result in several to choose from. Nearly all of them are concerned with two areas of the receiver
circuit.
First is the AVC circuit which many users feel has too short of a
release time. The second is the standard diode detector used in the
stock receiver which many users want to replace with a Product Detector.
The usual "ham mod" caveats should be seriously considered before
actually corrupting the original receiver circuitry. When the receiver
is functioning correctly and in good
alignment, the AVC does work adequately and so does the diode detector.
However, Collins designed the receiver in the late forties, long before SSB
transmissions became the standard voice communications mode.
>>>

>>> When operated as a typical late-forties communications
receiver, no serious problems will be encountered with the 51J Series receiver.
This means that you will have to reduce the RF Gain when receiving SSB or CW signals. If
signal quality is not an issue, the AVC can be left on but the RF Gain
will have to be reduced to about 8 or less for minimum distortion of a
typical SSB signal. Of course, signal strength will affect the RF Gain
setting for best demodulation. The AVC, the BFO injection and the diode
detector circuits are the primary reason that input signal level,
determined by the RF Gain, must be reduced so the proper ratio of BFO
injection to signal level will result in good SSB demodulation.
Additionally, when receiving CW signals with an older receiver,
excessive BFO injection into the detector can "mask" weak CW signals.
Maximum sensitivity in CW was always achieved by "riding" the RF Gain control
with the receiver out of AVC. Additionally, the Carrier Level meter will
no longer be useable for relative signal strength measurements when the
AVC is in the off position.

More on Mods -
The most common modification is to replace the 6BA6 BFO tube with a
6BE6 mixer tube and create a Product Detector for operation with the BFO
on - that is SSB and CW signal reception. This is a fairly simple
modification that if done carefully can be relatively easy to reverse
and put the receiver back to stock. I had a 51J-4 receiver that had this
modification and its performance wasn't very good. I felt the receiver
still distorted the SSB signal with the RF Gain at maximum (the object
of the mod.) I ended up removing the mod and returning the receiver to
stock configuration with much better performance operating the receiver
as a typical "late-forties" communications receiver. Additionally, the
51J-4 had the AVC mod that adds capacitance to increase the release
time. This was also returned to stock configuration. The person who
installed the mods did a very good job with no damage to the circuit
components that remained. This allowed an easy removal and rebuild to
stock for the BFO and AVC circuits. As an original 51J-4, the receiver
was one of my favorites but I unfortunately had to sell it in 2002.
Subsequently, in 2012, I was able to purchase another 51J-4 receiver.
This one was stock and had never been modified in its past. It's performance
is spectacular with plenty of sensitivity and with mechanical filters QRM is not an issue. After a complete IF/RF alignment it has become one of
my favorite receivers.

Certainly how you intend to use your 51J Series receiver will
determine your interest in any of the published mods. Remember that most
ham modifications will enhance one area of performance at the expense of
another. The product detector mods will work fine for SSB or CW and the
AVC mods allow for better SSB response but operating the stock receiver
in the manner in which it was designed will also give you great
performance in all modes of reception.

A Simple Mod for the 51J-1& J-2- There is one very simple modification that does improve the upper
audio frequency roll off. Across the primary of the audio output
transformer a .01uf capacitor is installed to keep the high frequency
transients down which also reduces the high frequency audio response to
about 2500Hz. That capacitor can be replaced with a .001uf cap to
increase the audio frequency response and still protect the primary from
transients. With the .001uf capacitor installed, you'll find the 51J-1
or J-2 receiver will sound a little more like a standard communications
receiver, that is, not high fidelity but not as "muffled" as it did
before. NOTE:
The R-388, the 51J-3 and 51J-4 all use a 6800pf (.0068uf) capacitor in
this application and don't really benefit from any change to a smaller
value capacitor.

Excessive End-Point Error
in the 70E-15 PTO -
The 70E-15 PTO end-points on nearly every
R-388 PTO are excessive and beyond the range of the adjustment
capacitor. Usually, if the EPE is more than 6.0kc, it will be out of the
range of the adjustment capacitor. All 70E-15 PTO end-point errors (EPE) are similar in that the
tuned range (1.0mc in ten turns) of the PTO has increased. I've never
encountered a 70E-15 PTO where the EPE actually has a decreased range.
The EPE issues are supposedly related to the questionable quality of the
ferrite slug material used in the 70E-15. However, I think the problem
is more a combination of the ferrite quality combined with continuous
operation with the resulting heat. This opinion is based on the acquisition of
an NOS 70E-15 PTO. This PTO had virtually no EPE probably
because it had spent the last 50 years stored in a box and had never
been subjected to the rigors of continuous operation. However, it is common
to find the 51J-4 receivers with virtually no EPE in the PTO and, of course,
51J-4 receivers post-date the R-388 receivers. Maybe the ferrite
problems were corrected in the later 70E-15 PTOs and the NOS example was
from this later manufacture. Once and a while 51J-4s will
be found with excessive EPE but these were probably in an environment
that required continuous operation. If your 51J-4 or R-388 has excessive
EPE, correction will require internal PTO compensation coil
trimming. This requires disassembly of the PTO. When trimming the
compensation coil, be sure to only remove one turn. If more than one
turn removal seems necessary in order to get the EPE in spec, you can
maybe remove another half of a turn - but no more. Excessive turn
removal will have the opposite effect and make the EPE totally
non-adjustable. Bill Orr wrote extensively about the R-388 and
correcting the 70E-15 PTO EPE problems in the form of an article in CQ
magazine.

The PTO Test Jig
for the 70E-15
- The earlier PTO, the 70E-7A, has the end-point adjustment capacitor
easily accessible on the top of the PTO can. It is rare that an early
PTO will have end-point errors that are not within the range of this
adjustment. The same cannot be said for the 70E-15 PTO. This later PTO
has chronic end-point error problems that are difficult to correct. Additionally,
the end point adjustment is in front of the PTO and is not accessible
unless the PTO is removed from the receiver. The end-point
adjustment capacitor is behind a hex-nut that has to be removed and then
there is a locking nut that also has to be loosened before the
adjustment capacitor can
be moved. There are some restorers that have built special
right-angle tools for accessing the locking nut and the adjustment
capacitor but these do require a lot patience to use. Generally, the PTO has to be removed from the receiver anyway
for trimming the compensation coil for end-point correction. I find it easier to just remove the PTO, do the
rework and the entire
adjustment on the bench with a good jig.
>>>

>>> The test jig can be easy to make. You just need to reproduce
the accuracy of the kilocycle dial to determine a 1000kc change in
frequency for exactly ten revolutions of the PTO shaft. A fixed reference line and a
very narrow
pointer is really all that is necessary. You shouldn't try to use an old
51J kilocycle dial because it will block easy access to the adjustment
capacitor. A very narrow pointer and an index scale that has a "zero"
line and several other lines on each side of "zero" will allow you to
see if the end-point error adjustment is proceeding in the right
direction. You'll have to count the ten turns but that's not difficult.
How accurate you observe your pointer and the index zero during the
adjustments will determine your overall accuracy when the PTO is
installed back in the receiver. Additionally, a really nice test jig might also have a built-in power
supply providing the 6.3vac and a regulated +150vdc necessary to
power-up the PTO. Binding posts or a cable with alligator clips could
provide the hook-up to the PTO. In this way, the PTO can be completely
removed from the receiver. It makes any rework (like compensation coil
trimming) much easier since you don't have to have the harness connected
to the receiver. However, unless you're going into the business of
70E-15 readjustments, just leave the PTO connected to the receiver and
provide enough bench space to rework the PTO with the receiver also on
the bench.

Rebuilding the Collins 51J-2 Receiver

Origin of the
Primary
51J-2 - I was initially given a cosmetically very nice 51J-2 by my
old ham friend, W7ZCA, Paul Eisenbath (now SK,) in exchange for
re-capping his Collins 75A-4 receiver. Paul had been given the 51J-2 at
sometime in the past and had done some rework on it. At one point, Paul
had the receiver setting on the floor of his workshop with the bottom RF
shield off, exposing the ten crystals of the Crystal Oscillator circuit.
Paul's young grandson, who was probably about 4 years old then, found
the temptation of the small shiny and removable crystals irresistible
and pulled all of them out of the holder and hid them around Paul's
workshop. This removal and hiding was all accomplished while Paul was
off in another room. In fact, it was several days before Paul even
noticed that the crystals were missing. Of course that was enough time for his grandson to forget where he had hidden all ten
crystals. Paul was able to find a few of the crystals but seven were
still missing. A
telephone call to International Crystal Manufacturing Company had surprising
results. After Paul told the sales person the story of how the crystals
came to be missing, they found the story so charming that they sent an
entire set of ten crystals for the 51J-2 to Paul - free of charge.

The crystals
got the 51J-2 working again but there were other problems that seemed
unfixable. The gearbox had severe wear that allowed the Megacycle
control to be advanced maybe three or four bands before the gearbox
would bind and jam up. You then had to rotate the Megacycle control back a band
or two to undo the binding and then proceed forward three or four more
bands. It was a cumbersome method but it did allow eventually getting to
the desired band. Additionally, the Antenna Coil primary on Band 2 was
open due to an excessive amount of RF accidentally injected into the
receiver while operating on 160 meters. Paul had obtained another
Antenna Coil but it was from an R-388 receiver (which doesn't have the
primary winding on the coil.)

So, that was the condition of the 51J-2 receiver when it was given to
me. I tried to use the receiver with the worn gearbox but it seemed like
sooner or later (probably sooner) it was going to break something in the
gear train. I decided to look for a parts set to rob the gearbox from to
correct the problem.

photo above: The 1950 Collins 51J-2
showing the metal dial bezel, the square illuminated Carrier Level
meter, the green "ham band" highlighting on the megacycle dial scale and the
Collins' winged emblem - all characteristics of the early 51J receivers.

Finding a
Suitable Parts Set - After a few months, a suitable 51J-2
showed up on eBay. Nobody seemed interested in it since it was in fairly
rough cosmetic condition. The operational condition was described as a
"fun radio" - whatever that meant. So, two hundred dollars later I was
the owner of another 51J-2 receiver. When the second 51J-2 arrived, I
found out what a "fun radio" was - totally and incompetently reworked
beside being non-functional
with a repainted front panel in black paint with white rub-on lettering.
I swear that the eBay photos were of a different front panel but by that
time the seller had pulled the auction photos. It really didn't matter because
all I wanted was the gearbox and that was in good shape.

The plan was
to use two 51J-2 receivers to build up one nice condition, fully
functional receiver. In checking over the two receivers, it seemed
easier to use the chassis of the second receiver as the starting point
and rebuild the entire receiver using the best parts from both units.
This allowed me to skip the tedious removal of the gearbox which
certainly would have resulted in many synchronizing problems.

The second receiver had been incompetently reworked and several of
the front end coils had broken wires that were their connections to the
trimmer capacitors. Also, all of the high quality tubular ceramic capacitors had been
replaced with cheap disk caps. I decided to strip out the receiver from
the front of the IF section back to the audio output and start over. I reinstalled
the high-quality tubular ceramic capacitors harvested from the first 51J-2. I
rebuilt the entire front-end of the receiver to repair the many broken
coil leads. I had to replace the third conversion input coil because it
looked like it had been burned. All parts used were either new
parts or good ones harvested from the first receiver.

Once the
electronic rework was complete, the front panel, the knobs, the
megacycle drum dial, the meter and many other parts were transferred
from the first 51J-2 to the new rebuilt receiver. I ended up with a
great looking, complete 51J-2 - but how did it work?

Troubleshooting - Upon power-up the new
rebuilt 51J-2 seemed to be working fine. I performed an alignment and
the receiver worked fine - or did it? After about 30 minutes of
operation the sensitivity dropped down to where signals barely moved the
carrier level meter. From the start, the audio would distort if the RF
Gain control was higher than 8 when in AVC. Clearly, there were a few
more bugs to work out before the receiver could be called "complete and
working."
However, other projects came into the shop and the 51J-2 was put on the
shelf and temporarily forgotten.

In fact, "temporary" ended up being about three years before I got back to the receiver.
I had recently been told that Paul W7ZCA had become an SK and that had
me reflecting about all of the "deals" we had exchanged over the years. I
remembered the 51J-2 and thought that I should probably finish that
receiver since it was such a nice looking example of a fairly rare
Collins.

The strangest problem was the erratic variable gain of the receiver.
When on the bench in the normal position the gain was down but when the
receiver was placed up on its side to troubleshoot the gain would then
be somewhat normal. I could even tilt the receiver about 20 degrees and
get the gain to go up and down with just a slight change of the angle -
weird. >>>

>>>To narrow the problem down I used a clip lead type scope probe
and measured at various points in the receiver while tilting the
receiver back and forth on the bench. I had to inject a fixed-level
signal into the receiver's antenna input so I knew the variations in the
gain were occurring because of a fault and not the signal level
changing. I could see the gain change problem was occurring just past
the first IF amplifier tube. All components and the tubes had already either
been checked or replaced except the IF transformer itself. I removed the
IF transformer cover and discovered that the coil and the ferrite
shields were at the bottom of the mount and loose. Apparently, moving
the receiver around changed the position of the IF coil which changed
the coupling and resultant gain. Upon removing all of the IF transformer
covers, it was discovered that every IF transformer had loose coils,
loose ferrite shields or both. Complete removal and rebuilding of all of
the IF transformers was necessary. I used epoxy to secure the ferrite
shields in position. I actually used the IF transformers from the first
51J-2 because they were in better overall condition but still required
securing of the ferrite shields to prevent future problems. Upon power-up, I had more gain through the 51J-2
than ever before and an IF alignment only improved the response.

photo above: A 51J-2 IF Transformer showing the
coil structure beneath the ferrite shields. Note that this
transformer had the mica caps replaced but nothing was done to
secure the ferrite shields.

photo above: The 51J-2 after rebuilding

Although I thought that the IF transformer problem solved everything
that was wrong with the 51J-2, I soon discovered that the AVC would
cause distortion after several minutes of operation. I had already
replaced all of the tubes in the receiver with a set that checked fine
on the TV-7 tube tester. However, you can't find all tube problems with a
tube tester. Once the AVC started to cause the distortion, I replaced
the AVC amplifier tube with no change in the problem but changing the
Dectector/AVC Rectifier tube cleared up the problem. I had already
rebuilt that entire section of the receiver so I was sure the problem
had to be tube related.

An additional problem was only on the AM BC band or Band 1. Low gain was
the result of a defective mixer coil that appeared to have gotten very
hot. Fortunately, the first 51J-2 receiver provided a good condition
coil. These
coils are easy to replace as they are only held in place by two tangs
that have to be pushed inward to dismount a coil and "snap" into place
when installing the coil into the mounting hole. The wire leads are the
same diameter as the coil wire itself and the connections to the trimmer
capacitors is fairly delicate so care must be taken in the removal and
installation.

A full IF/RF alignment had already been performed early on the receiver
and a quick touch-up was all that was necessary for top performance that
is at or better than the original specs. Audio is very nice as I did
replace the .01uf across the audio output transformer with a .001uf to
increase the high frequency audio response. I am running the receiver
into a 12" 4 Z ohm speaker in a large cabinet and the response in very
nice with some bass on SW BC stations and powerful ham AM stations along
with enough highs to significantly reduce the normally "muffled"
audio quality.

Photographic Details on the 51J-2

photo left: The top of the chassis of a 51J-2 receiver showing
the earlier 70E-7A PTO. Under the metal shield on the PTO is a metal
octal 6SJ7 tube. In front of the PTO tube is the glass holder for the
desiccant that protects the PTO from moisture ingression. Otherwise the receiver is very similar to the
later 51J-3 and R-388 receivers. Photo above:
Close-up of the 70E-7A PTO

photo above: The back of the
51J-2 showing the unique slanted top cover that is used on all of the
51J Series receivers. The top cover must be installed even if the
receiver is installed into a cabinet. The top cover shielding eliminates
some spurious oscillations that will be tuned at various frequencies
throughout the tuning range. Note the ID plate - this is NOT standard.
The ID information is typically silk-screened to the rear of the
chassis. This particular 51J-2 had holes drilled thru the information so
a suitable "data plate" was made from a derelict "parts set" 51J-2 chassis.

photo above: The underside of the a 51J-2 receiver
showing the smaller bottom shield that only covers the front-end of the
receiver. Note that some of the capacitors have been replaced on this
51J-2 while the .01uf tubular ceramic capacitors are still in place.

photo above: One common problem encountered on the
51J-2 receiver is inside the IF transformers. The entire lower coil and
ferrite shield have
loosened and have slid down to the bottom of the transformer. This
reduces coupling and output level of the transformer. To
correct, the IF transformer must be removed, the
coils put back into their proper position and then secured with epoxy to
prevent future movement. Rough handling of the receiver such as
"slamming" in down on a work bench or dropping it from a foot or so down
onto the pavement at a swap meet is the probable cause.

Rebuilding the R-388/URR ReceiverThe R-388 is the most commonly encountered version of
the 51J series of receivers. Many thousands were built for the military
over a period from 1950 up to 1962. As a result, parts
are fairly easy to find which makes a rebuild easier to accomplish.
However, just because there are lots of spare parts and parts sets
around doesn't mean that the parts needed will be cheap - they aren't.
Mainly because of the "Collins" name but also because any parts taken from
any R-388 almost relegates that receiver to "not restorable" status which can be a
difficult decision for the owner to make. The following "basket case" is
not typical of that status since most of the parts were present. Just
the receiver was mostly disassembled.

The Basket Case
- A friend of mine had the luck to find three R-388 receivers locally.
They had been listed on Craig's List. His telephone call to me was
mainly for advice on how to "power-up" the three receivers. I told him
that typically R-388s will pretty much work "as found" but it would be a
good idea to thoroughly check the filter capacitor and to test all of
the tubes before applying AC. This was accomplished but he found that
none of the receivers seemed to work. I was asked if I would "take a
look" at them. We made a deal that I would service and align the two
best receivers in trade for the worst of the bunch which by this time
had been disassembled for parts needed in the other two receivers.

I
delivered the two serviced and aligned R-388s and picked up the "parts
set" R-388 while at a ham swap meet in Reno. What I got was the
R-388 chassis, the front panel and a box of parts. The gearbox and the
main band switch had become de-synchronized during some of the
disassembly. Also, about half of the tubes were missing as were nearly
all of the screws. I was able to have the screws sent to me through the mail. I had
plenty of tubes so that wasn't a problem. Really, all that was necessary
was to put the R-388 back together and go from there.

The front panel
was in exceptional condition and had been cleaned by the former owner.
The kilocycle glass was missing but I had a glass piece from a "parts
set" 51J-2 that fit correctly. When I purchased a "parts set" 51J-2 to
complete the rebuild of another 51J-2 receiver I was lucky enough that the "parts
set" 51J-2
that had an almost perfect R-388 megacycle drum installed. Fortunately,
the original 51J-2 megacycle drum was excellent so I really didn't need
the R-388 one that came with the "part set." It was so nice though I
saved it and wrapped it up in plastic to protect it. So, here it was a
few years later and I was rebuilding an R-388 that had a very well-worn
megacycle drum. Obviously, that saved megacycle drum had "saved the day."

photo above: The R-388 chassis after restoration.
Note the megacycle drum - this is an original drum scale, not an overlay.
Note the plastic bag piece on the Crystal Filter housing. These
bags usually contained some spare parts, connectors, etc. The alignment
tools are reproductions.

Synchronizing the Band
Switch to the Gear Box - This is really much easier than
it sounds. Since the gear box has mechanical stops at each end all
that is necessary is to know which way the Megacycle knob is rotated
to change bands in the proper direction. Since counterclockwise
rotation increases the frequency, rotation clockwise to the end stop
would be the .5 to 1.5 mc band or Band 1. Once Band 1 is in position
then a check of S-106 can be visually accomplished to see where the arm of the switch is. By
checking the schematic versus looking at S-106 it was determined that
the switch was actually on Band 2 and all that was necessary was to
loosen the set screws on the coupler from the gear box to the band
switch and then rotate the switch shaft until Band 1 was in position and
then tighten the coupler set screws. Rotating through all thirty band
positions did verify that the switch rotated from position 1 to position
16 and then from Band 17 to Band 30 the switch shaft did not change
position. Then when returning from Band 30 down to Band 17 no change in
the switch shaft position but with Band 16 on down to Band 1 the switch shaft did
rotate correctly and stop in position one. While doing these tests all
of the other movements of the slugs and slug racks were verified and no
problems were noted.

Dial
Drum Drive Cable and MC Dial Pointer Cable - These
are special metal cables that have a plastic covering over them.
Luckily, both cables were present but were loose in the box of
parts. I think the cable material is available from various Collins
suppliers and the dimensions and installation instructions are in
the manual if your R-388 is missing these cables.

A Different
Carrier Level Meter- Most R-388 receivers will have
a Burlington sealed carrier level meter. These high quality metes
have a metal housing and metal scale. The meter installed in
this R-388 was built by Marion Electric Instrument Company. Probably
a field replacement. It
has a convex face with gloss finish mounting flange with a sealed
metal housing.

Repro ID Tag
- As can be seen, this receiver originally had two data plates installed
that had been removed in the past. The
center tag was the serial number identification plate and the tag to the
right was a "Caution" tag that informed the user to refer to the TM
manual. Luckily, there are a few repro tags available from Mike Chanter
(Collins Radio Association.) He supplies a 1950, 1953-54 and 1957 type
tags. Since this R-388 was probably built on a 1951 contract, I should
have gotten the 1954 tag but instead I bought the 1950 version. The
thinking was that the MFP date of "JAN 52" would have to be from
an earlier build, thus the 1950 tag. However, knowing that Collins built
R-388 receivers an put them into inventory for later contract sales,
maybe the 1953-54 tag would have been a better choice.

Knobs and Tubes -
Since this R-388 was a "basket case" it naturally didn't include the
knobs or many of the tubes. Luckily, I had a complete set of Daka-Ware
knobs that had come with the "parts set" 51J-2 receiver. I did have
to search through the junk box to find a set of Daka-Ware kilocycle
and megacycle skirted knobs since those weren't on the 51J-2 "parts
set." Most of the tubes missing were the 6BA6 tubes. In fact, the R-388
uses seven 6BA6 tubes. Also, missing were the three 6BE6 tubes, the 6AQ5
and the 5V4 rectifier. All tubes were found in the tube "junk" boxes. All tubes used in the R-388 should test well
above "minimum acceptable" for best performance.

Restoring the Dial Bezel -
The dial bezel had been broken at one time and glued together to repair.
The break was at the bottom of the bezel - an area hidden by the skirt
of the main tuning dial. There was also a chip in the opening for the
kilocycle dial glass. I decided to use an epoxy fill to repair the break
because there were some missing sections. Tape dams were used and epoxy
was layered to complete the fills. After the epoxy had set-up the areas
were leveled by filing and then polished. Usually I would have used
black power coloring for the epoxy but I didn't have any around so I
carefully applied jet black nitrocellulose lacquer and polished this to
match the bezel bakelite. To finish the bezel required filling the
engraving "MEGACYCLES" and "KILOCYCLES" along with the "tick marks" for
the kilocycle index. I used Artist's Acrylic mixed to a manila color and
applied with a brush. The fill paint is left to set up for one minute
and then a paper towel piece dampened with Glass Plus is used to remove
the excess fill paint. The end result was a bezel that looked great and
had repairs that were very difficult to see.

New Power Cord
- The original power cord had been cut on this R-388/URR. The
plastic strain relief was broken when trying to remove it.
Fortunately, most larger hardware stores carry the correct type of
power cable and also the same style of plastic strain relief.

photo left: The R-388 immediately after restoration.
See below for updated photo of this receiver

IF and RF Alignment
- The alignment procedure in the TM has a few steps that are outdated.
This is because today we have easy access to digital frequency counters
that are extremely accurate. When the TM directs you to use the R-388's
Crystal Calibrator to "beat" against the signal generator's 500kc input
to be sure that it is accurate, it's because back in 1950 that was the
most accurate way to assure that 500kc was the input signal. Today's
digital frequency counter (or synthesized signal generator) is just as
accurate and much easier to use. I like to check the Crystal Filter's
crystal frequency just to be sure. Usually that crystal is within 0.01kc or so of 500kc but I use that frequency of the crystal as
the IF. This assures that the Crystal Filter works great (and they will
-
if you take care in the IF alignment.) As to why the TM directs you to
"load down" the IF transformers with an RC load consisting of a .01uf
capacitor and a 47K resistor in series, I believe that was to assure
that the alignment tech wasn't going to overload the IF with too much
signal and therefore slightly misalign the IF because the input signal
was so broad and strong. Keep the IF input signal
level low and do the IF alignment with the Crystal Filter on and in
position 4 and your alignment will be accurate. You can install
the load if you want too but I don't think it's necessary if you watch
the level of the signal generator carefully. Though the special
alignment tools (that are always missing) will help with the alignment
they are not strictly necessary. Reproduction tools are available so you
can obtain a set and try them out. I think the original alignment tools
make the trimmer caps easier to adjust but not the slugs. The IF is
easier adjust with the proper alignment tool, also.

Variable IF and RF Alignment -
Signal is injected into the Antenna Input SO-239 connector. Align the
Variable IF sections first. There are two sections, odd and even,
requiring different frequency inputs. Alignment is standard in that the
trimmer caps adjust the high end of the range and the slugs (inductance)
adjusts the lower end.

RF tracking is straight forward but you will only have to adjust six
ranges as the remainder of the bands are tracked by heterodyne action.

Mechanical set-up of the kilocycle dial and the PTO should have been
performed before the tracking alignments. All 70E-15 PTOs will have some
end-point error unless they are removed and calibrated. If you are
satisfied with the EPE and it happens to be a few kilocycles then be
sure to use the peak of the receiver output at each calibration
frequency rather than the actual dial readout. For example, if you have
to align to 7.40kc but the actual peak output is on 7.402kc, then use
7.402kc readout on the receiver and 7.40kc input on the RF generator and
then adjust the proper trimmers or slugs for peak output. The more
end-point error there is in the PTO the further out this procedure
becomes but for just a couple of kilocycles of error it works fine. The
correct procedure would be to rework the PTO for zero error and then all
of the tracking will be accurate.

R-388/URR Performance
- I've worked on many R-388s and this of course includes many
full alignments. Probably the most important improvement that can be
performed on the R-388 receiver is to do a full alignment. It's
surprising how far out the alignment can be and the receiver still seems to be
performing fine. So, this evaluation of R-388 performance is for
receivers that are in good condition and have had a recent full IF and RF
alignment.

The R-388 is probably the overall best performer of the
entire 51J Series for vintage AM ham stations. This is because it doesn't have the mechanical
filters that were installed in the 51J-4 receiver. Though the MFs
are great for SSB and CW, the 6.0kc filter is not particularly ideal
for AM reception. This is because Collins' engineers were thinking
"communications" (and primarily data communications) and
not wide-bandwidth audio reproduction. The R-388 relies on the
designed-in bandwidth of the fixed IF section of the receiver which is probably around
8.0kc at 3db down or a bit wider.
However, the IF passband curve is more bell-shaped rather than the steep slopes of
the Collins' mechanical filters. So, the first "plus" is the IF
bandwidth of the R-388 for AM reception. The audio output 6AQ5 does
have a capacitor from plate to B+ for impedance matching which on
the earlier J-1 and J-2 receivers seriously limited the upper audio
response but the R-388 changed the value of this capacitor to 6800pf
which doesn't affect the audio response. So, you should expect your
R-388 to sound just a little limited in the upper end of the audio
range but most listeners find this audio to be "pleasingly mellow."

Dial accuracy is legendary and is plus or minus one kilocycle if the
receiver is calibrated to the nearest 100kc point on the particular
band in use. Ultimate accuracy is limited by the 70E-15 PTO which is
also legendary for its end-point error problems. Most R-388s will
have around 6kc of EPE which can usually be adjusted out. Greater
EPE requires PTO rework to correct. When the EPE is adjusted out,
the receiver is dead-on accurate. Unfortunately, the 70E-15 PTO must
be out of the receiver to perform the EPE adjustments. There are
some special tools that have been built by enthusiasts that allow
the adjustments to be accomplished without PTO removal but they are
difficult to use.

Stability is solid and
drift-free. Sensitivity is competitive. As a comparison, one might find that the
R-390A is a bit better at digging out signals especially if the
R-390A has been rebuilt and aligned. Still, with a good antenna, the
R-388 will hear just about any signal on the band.

Selectivity is
entirely controlled by the use of the Crystal Filter. Why many hams
refuse to use Crystal Filters is a mystery. CFs really do a good job at
reducing QRM. On AM, select position 1 or 2 and adjust the Phasing
for minimum bandwidth. Then tune the AM signal "on the noise."
You'll find most of the interference is gone or greatly reduced. If
adjacent frequency SSB activity is a problem then adjust the Phasing
slowly and you'll find a point where the dominant audio frequency of
the SSB QRM is attenuated substantially. Use the same procedure for SSB
reception. For CW, with the CF on you can tune the CW signal off of
zero slightly to find the "peak" response. The peak response can
also be adjusted with the Phasing control and slight tuning of the
signal. The Phasing control can be used to reduce specific audio
tone frequencies, that is, to eliminate
AM heterodynes. Heterodynes were quite common when there were a lot of
AM signals on the ham bands but, nowadays, most AM activity is on
nets and one doesn't usually encounter heterodynes too often.
On CW, the CF can reduce interference from a nearby CW signal by this heterodyne elimination method. When trying
to eliminate CW QRM or an AM heterodyne adjust the Phasing control
slowly. The "notch" is very narrow and it can be easy to miss its
effect on the interference if the Phasing is adjusted carelessly.

Set-up for station receiver operation is somewhat involved since the Break-in function does
require a separate +12vdc power supply that is switched with the
transmitter's antenna relay auxiliary contacts. I built a very small
+12vdc power supply housed in a metal box. It can be switched on by
connecting the AC input to the same transmitter terminals that drive
the dow-key relay. Upon transmit, the +12vdc is actuated along with
the dow-key and the receiver muted. Or, if the transmitter has a
built-in antenna relay with only auxiliary contacts available (like
the ART-13) then the +12vdc power supply AC can be connected to the
120vac line and the +12vdc output connected to the auxiliary NO
contacts of the transmitter relay. Closure of the contacts connects
+12vdc to the receiver to actuate the muting relay. Remember to
always provide the Break-in function for the R-388 as this
disconnects the antenna within the receiver and protects the input
circuits from excessive RF levels.

Better audio is obtained from the
4.0 Z ohm output with a matching Z speaker. The 600 Z ohm output was
for driving data devices and requires a matching transformer to operate a
standard Z speaker.

Though the R-388 will "pick-up" signals with a
minimal antenna best performance is with a full-size, impedance-matched
(tuned) antenna. With a first-class antenna, the R-388 will hear
everything that is on the air with conditions being the only
limitation.

UPDATE: July
17, 2016 - As restorers, we're never, ever quite through with a
project. We're always keeping an "eye out" for parts that would
either enhance our restorations, or, to maybe add a "missing part."
This was the case with the R-388/URR profiled above.

Our first find was to add the original data plate,...not an
original,... THE original. I was able to talk the fellow who had
removed it into trading the original data plate for the reproduction tag I had
on the receiver. Now the data plate reflects that this R-388 is from
a 1951 contract.

The second find was probably a year later when a nice condition
Burlington carrier level meter showed up on eBay at a reasonable BIN
price. It was acquired, restored and installed.

About six months later at a local ham swap meet there was a
"FREE" R-388 front panel. It was in very rough condition and missing
almost everything but it had nice original grab handles and a
perfect escutcheon. These pieces were removed, cleaned up and then
installed on my R-388 (note in the earlier photo above that the grab handles
are "close" but not correct.)

The photo to the right shows the R-388 after these parts were
installed - July 17, 2016.

Now, if I can just find the "Warning" tag that mounts next to the
data plate.

Aligning the 51J-4 Receiver - Performance Evaluation

IF Alignment
with Mechanical Filters
- Since the 51J-4 uses mechanical filters to define the IF passband,
it is essential that the IF be centered for the mechanical filters
and the crystal filter. Usually an accurate setting of 500kc on the
signal generator is okay but it's better to use the
crystal filter crystal frequency to set up the IF on the signal
generator. With the Crystal Filter on and set to position 3 or 4 and
mechanical filter 3.1kc selected, sweep through the IF and note
where the crystal "peaks." Usually it will be very close to 500kc -
within 0.01 or 0.02kc of 500kc is normal. Note on the digital
frequency counter the exact frequency and then make sure that when
performing the IF adjustments that the signal generator has not drifted
or changed frequency. Again, you must keep the signal generator
frequency at the crystal frequency exactly! It's a good idea to do
the adjustments to the IF transformers with the Crystal Filter
turned on and set to position 3 or 4 and with the Phasing set to the
narrowest response. Monitor the Diode Load output with an analog VTVM and keep the signal generator output at a level that produces
about -7.0vdc on the Diode Load. Connect the signal generator
through a .01uf capacitor to the grid of the last Mixer V-107. The
manual indicates that a RC load consisting of a .01uf cap and a 47K
resistor needs to be connected to the IF transformers as they are
adjusted but I don't bother since we are carefully and accurately
controlling both the frequency input and the signal level into the
IF section of the receiver. Peak all of the IF transformers top and
bottom and also the adjustment on the mechanical filter assembly.
Recheck to be sure that all adjustments are at peak at the
correct input frequency. Disconnect the signal generator and the VTVM.

Test the IF alignment by tuning through a broad AM signal
with the 6.0kc filter selected. Carefully listen to the signal as
you tune from one end to the other. You should hear the AVC attack
at the low end of the signal's passband, then the signal will clear up and as you tune
through about 6kc or so, the signal should stay constant and clean.
You will hear the AM signal narrow in audio bandwidth as you
approach the center of the 6.0kc filter but the signal level should
remain constant with no distortion. As you tune out of the high end of the signal's passband the AVC will react to the noise as the signal is reduced by the steep
slope of the mechanical filter. This should be repeated for the
3.1kc filter and the 1.4kc filter also.

Other than the importance of the centering of the IF for the
mechanical filters, the 51J-4 aligns just like the R-388. The
procedure in the 51J-4 manual is somewhat different but the receiver
is exactly the same as the R-388 with the exception of the
mechanical filters so the R-388 procedure can be used , if
necessary.

photo above: The top of the chassis on a 51J-4
receiver. Note the mechanical filter assembly to the right and
partially above the PTO. The three mechanical filters are the
plug-in rectangular units.

NOTE: This 51J-4 now belongs to KB6SCO

photo above: Under the 51J-4 receiver

51J-4 Performance
- For dial accuracy, stability and selectivity, it's hard to beat the 51J-4.
Mechanical filters provide a superior selective ability to all but eliminate
most QRM.
Sensitivity is very good or at least competitive. Where the 51J-4
has problems is in its audio reproduction. To say the receiver
sounds awful on AM wouldn't be too much of an exaggeration for an audio
purist. I've owned two 51J-4 receivers and both of them sounded
pretty much the same on AM. The AM signals are usually tuned with
the 6.0 kc mechanical filter selected. What is noticed is that on
either side of the passband center the audio range is pretty good but at
passband center
the audio is "muffled." This is because the 6.0 kc filter is limiting
the audio response to 3.0 kc at the highest. On either side of
passband center
you are tuning to one sideband or the other of the AM signal and the
audio response can go up to 6kc or so and thus the audio highs sound
better. This is normal for the 51J-4. In fact, Collins recommended
that when tuning AM to use one sideband or the other depending on
receiving conditions. On SSB, the 51J-4 works quite well if it is
operated as a typical early fifties receiver without a product
detector. The same goes for CW reception. Of course, the 3.1 kc and
the 1.4 kc filters can be used in those modes for greater selectivity. You can get away with
the 3.1 kc filter in AM if QRM is particularly bad by selecting one
sideband or the other depending on where the QRM is. Those who are
75A-4 users probably wonder why the Crystal Filter continued to be
used when MFs are provided. The Crystal Filter is actually better
for certain kinds of QRM, especially heterodynes or adjacent
frequency SSB signals. The Crystal Filter Phasing can eliminate that
kind of interference where the MFs can't. For SW BC the
51J-4 is okay but still the limits on the audio reproduction have
most of the SW BC sounding pretty bad. Remember, the 51J-4 was
primarily for communications and the basic 51J design was for data
reception, not voice. The dial accuracy meant laboratories didn't
have to have frequency meters anymore. Same with many of the
military installations. Dial accuracy, frequency stability and excellent selectivity was
what most users were interested in when purchasing a 51J-4. Great
audio reproduction was not even considered since the passband had to
be kept narrow for selectivity.

Conclusion
- All Collins enthusiasts have their favorite models. For me, the
51J Series has the best of what I like - good performance
characteristics with an incredible "bench presence." Add to that,
superior build-quality resulting in excellent reliability and you
end up with a really great receiver. The earlier 51J-1 and J-2 have
desirable features and performance but, lucky for those amateurs and
collectors who want the best, the most common version really has
those features that work for those users. Of course, that receiver
version is the R-388/URR. Thousands built in the early fifties - a
time when vacuum tube design was peaking and the military demanded
the very best radio gear. The best communications equipment design
and manufacturing company at the time, Collins Radio Company,
provided a real "winner" in the 51J Series and especially with the
R-388/URR.

References:

1. Nick England's great website
www.navy-radio.com has a very
detailed section on the R-388/URR receiver and all of the
variations. Also includes some info on the 51J-4.

2. Original Manuals for 51J-2, 51J-4, R-388/URR

3. Thanks to all of the 51J Series enthusiasts for conversations,
information and anecdotes that have been exchanged "on the air," online
and in person.

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